A rotary compression molding apparatus is typically employed for the manufacture of molded plastic closures. A rotary turret carries multiple, vertically oriented tool assemblies which are rotated by the turret relative to upper and lower fixed cams. Rotary motion of the tooling relatively moves respective sets of upper and lower or, male and female, mold assemblies. As the turret rotates, a metered charge of molten plastic is placed into each open female mold, and the male and female molds relatively move to compress the molten plastic therebetween to form the closure. Liquid cooling within the tooling promotes rapid plastic solidification. The molding cycle is completed by relative movement of the tooling to open the mold cavity, and eject the molded closure.
Quality monitoring or inspection techniques employed after compression molding have necessarily resulted in a lag time between identification of molding problems and their correction. By one inspection technique, molded closures are sampled, and carefully measured. As a result of the time lag between identification of a problem and its correction, many unacceptable closures may be produced. It is also possible that poor quality closures can be manufactured between the times at which samples are taken. Additionally, the detection of a poor quality closure does not necessarily identify the specific problem that resulted in its formation, thus requiring secondary measurements and process experiments to determine the cause of the faults. Examples of secondary measurements and process experiments would be dimensional measurements obtained from a coordinate measuring machine or a caliper.
Another technique for inspecting molded closure parts employs a vision-based inspection system, which visually inspect either periodic closure samples, or 100% of the closures being molded. However, these systems have proven to be expensive to implement in connection with high speed production, which may entail hundreds of closures per minute. Such vision-based systems are sensitive to the background lighting of the room that the apparatus is in and process lighting angles. While the time lag between manufacturing and inspection is minimal, detection of poor quality closures does not give specific information regarding the cause of the problem.
An inspection or monitoring system is needed wherein measurement of molded closures can be effected on an on-line basis as they are ejected from an associated compression molding apparatus. The testing would be conducted in a fashion such that each individual closure can be associated with a particular one of the mold tooling sets of the molding apparatus. Problems associated with the specific tool set can thus be readily identified. Furthermore, by monitoring specific parameters, specific sub-systems and process settings of the molding apparatus could be modified. For example, temperature measurement of each closure correlates to dimensional shifts, quality shifts, and cooling system performance. Measurement of top panel thickness correlates directly to closure weight and final dimensions. Warpage indicators or the concavity of the closure top surface, correlate directly to cooling flow and plastic melt temperature.
Combinations of measurements' behavior would allow technicians to quickly diagnose a problem and point to a sub-system of the molder, or individual tool that would require maintenance or adjustment. Because the measurements would be variables, rather than attributes, they would lend themselves to control charting, and would indicate processes that were changing, thus giving early warning of changing processes and allowing maintenance and adjustment to be performed before scrap is produced. This would be in direct distinction from the previous methods, which are only triggered when scrap is produced.
The primary object of the present invention is to provide a method and apparatus for monitoring molded closures that would measure specific parameters of the closure so that problems with specific tool sets, sub-systems, and process settings of the molding apparatus could be identified, thus substantially reducing the amount of scrap that is produced.